[Messenger] Added documentation about DispatchAfterCurrentBusMiddleware

messenger/message-recorder.rst

@@ -0,0 +1,192 @@
+.. index::
+    single: Messenger; Record messages; Transaction messages
+
+Transactional Messages: Handle Events After CommandHandler is Done
+==================================================================
+
+A message handler can ``dispatch`` new messages during execution, to either the same or
+a different bus (if the application has `multiple buses </messenger/multiple_buses>`_).
+Any errors or exceptions that occur during this process can have unintended consequences,
+such as:
+
+- If using the ``DoctrineTransactionMiddleware`` and a dispatched message throws an exception,
+  then any database transactions in the original handler will be rolled back.
+- If the message is dispatched to a different bus, then the dispatched message can still
+  be handled even if the original handler encounters an exception.
+
+An Example ``RegisterUser`` Process
+-----------------------------------
+
+Let's take the example of an application with both a *command* and an *event* bus. The application
+dispatches a command named ``RegisterUser`` to the command bus. The command is handled by the
+``RegisterUserHandler`` which creates a ``User`` object, stores that object to a database and
+dispatches a ``UserRegistered`` event to the event bus.
+
+There are many subscribers to the ``UserRegistered`` event, one subscriber may send
+a welcome email to the new user. We are using the ``DoctrineTransactionMiddleware``
+to wrap all database queries in one database transaction.
+
+**Problem 1:** If an exception is thrown when sending the welcome email, then the user
+will not be created because the ``DoctrineTransactionMiddleware`` will rollback the
+Doctrine transaction, in which the user has been created.
+
+**Problem 2:** If an exception is thrown when saving the user to the database, the welcome
+email is still sent because it is handled asynchronously.
+
+``DispatchAfterCurrentBusMiddleware`` Middleware
+------------------------------------------------
+
+For many applications, the desired behavior is to have any messages dispatched by the handler
+to `only` be handled after the handler finishes. This can be by using the
+``DispatchAfterCurrentBusMiddleware`` middleware and adding a ``DispatchAfterCurrentBusStamp``
+stamp to `the message Envelope </components/messenger#adding-metadata-to-messages-envelopes>`_.
+
+Referencing the above example, this means that the ``UserRegistered`` event would not be handled
+until *after* the ``RegisterUserHandler`` had completed and the new ``User`` was persisted to the
+database. If the ``RegisterUserHandler`` encounters an exception, the ``UserRegistered`` event will
+never be handled and if an exception is thrown while sending the welcome email, the Doctrine
+transaction will not be rolled back.
+
+The  ``dispatch_after_current_bus`` middleware is enabled by default. It is configured as the
+first middleware on all busses. When doing a highly custom or special configuration, then make
+sure ``dispatch_after_current_bus`` is registered before ``doctrine_transaction``
+in the middleware chain.
+
+**Note:** The ``dispatch_after_current_bus`` middleware must be loaded for *all* of the
+buses. For the example, the middleware must be loaded for both the command and event bus.
+
+.. configuration-block::
+
+    .. code-block:: yaml
+
+        # config/packages/messenger.yaml
+        framework:
+            messenger:
+                default_bus: messenger.bus.command
+
+                buses:
+                    messenger.bus.command:
+                        middleware:
+                            - validation
+                            - doctrine_transaction
+                    messenger.bus.event:
+                        default_middleware: allow_no_handlers
+                        middleware:
+                            - validation
+                            - doctrine_transaction
+
+    .. code-block:: xml
+
+        <!-- config/packages/messenger.xml -->
+        <?xml version="1.0" encoding="UTF-8" ?>
+        <container xmlns="http://symfony.com/schema/dic/services"
+            xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+            xmlns:framework="http://symfony.com/schema/dic/symfony"
+            xsi:schemaLocation="http://symfony.com/schema/dic/services
+                https://symfony.com/schema/dic/services/services-1.0.xsd">
+
+            <framework:config>
+                <framework:messenger default_bus="messenger.bus.command">
+                  <framework:bus name="messenger.bus.command">
+                      <framework:middleware id="validation">
+                      <framework:middleware id="doctrine_transaction">
+                  </framework:bus>
+                  <framework:bus name="messenger.bus.command" default_middleware="allow_no_handlers">
+                      <framework:middleware id="validation">
+                      <framework:middleware id="doctrine_transaction">
+                  </framework:bus>
+                </framework:messenger>
+            </framework:config>
+        </container>
+
+    .. code-block:: php
+
+        // config/packages/messenger.php
+        $container->loadFromExtension('framework', [
+            'messenger' => [
+                'default_bus' => 'messenger.bus.command',
+                'buses' => [
+                    'messenger.bus.command' => [
+                        'middleware' => ['validation', 'doctrine_transaction'],
+                    ],
+                    'messenger.bus.event' => [
+                        'default_middleware' => 'allow_no_handlers',
+                        'middleware' => ['validation', 'doctrine_transaction'],
+                    ],
+                ],
+            ],
+        ]);
+
+.. code-block:: php
+
+    namespace App\Messenger\CommandHandler;
+
+    use App\Entity\User;
+    use App\Messenger\Command\RegisterUser;
+    use App\Messenger\Event\UserRegistered;
+    use Doctrine\ORM\EntityManagerInterface;
+    use Symfony\Component\Messenger\Envelope;
+    use Symfony\Component\Messenger\Stamp\DispatchAfterCurrentBusStamp;
+    use Symfony\Component\Messenger\MessageBusInterface;
+
+    class RegisterUserHandler
+    {
+        private $eventBus;
+        private $em;
+
+        public function __construct(MessageBusInterface $eventBus, EntityManagerInterface $em)
+        {
+            $this->eventBus = $eventBus;
+            $this->em = $em;
+        }
+
+        public function __invoke(RegisterUser $command)
+        {
+            $user = new User($command->getUuid(), $command->getName(), $command->getEmail());
+            $this->em->persist($user);
+
+            // The DispatchAfterCurrentBusStamp marks the event message to be handled
+            // only if this handler does not throw an exception.
+
+            $event = new UserRegistered($command->getUuid());
+            $this->eventBus->dispatch(
+                (new Envelope($event))
+                    ->with(new DispatchAfterCurrentBusStamp())
+            );
+        }
+    }
+
+.. code-block:: php
+
+    namespace App\Messenger\EventSubscriber;
+
+    use App\Entity\User;
+    use App\Messenger\Event\UserRegistered;
+    use Doctrine\ORM\EntityManagerInterface;
+    use Symfony\Component\Mailer\MailerInterface;
+    use Symfony\Component\Mime\RawMessage;
+
+    class WhenUserRegisteredThenSendWelcomeEmail
+    {
+        private $mailer;
+        private $em;
+
+        public function __construct(MailerInterface $mailer, EntityManagerInterface $em)
+        {
+            $this->mailer = $mailer;
+            $this->em = $em;
+        }
+
+        public function __invoke(UserRegistered $event)
+        {
+            $user = $this->em->getRepository(User::class)->find(new User($event->getUuid()));
+
+            $this->mailer->send(new RawMessage('Welcome '.$user->getFirstName()));
+        }
+    }
+
+.. note::
+
+    If ``WhenUserRegisteredThenSendWelcomeEmail`` throws an exception, that exception
+    will be wrapped into a ``DelayedMessageHandlingException``. Using ``DelayedMessageHandlingException::getExceptions``
+    will give you all exceptions that are thrown while handing a message with the ``DispatchAfterCurrentBusStamp``.